Method of improving high temperature stability of PTSI/SI structure
BACKGROUND OF THE INVENTION 
     The present invention relates to a method of improving high temperature stability of platinum silicide (hereinafter referred as PtSi) film on a silicon substrate (hereinafter referred as a PtSi/Si structure), and in particular relates to a method of improving high temperature stability of a PtSi/Si structure by the implantation of fluorine-containing ions. 
     Platinum silicide (PtSi) has been widely used in silicon based devices and integrated circuits. It forms stable and reproducible Schottky contact on lightly doped n-type silicon and ohmic contact on p-type silicon at room temperature and heavily doped n-type silicon. The PtSi/p-Si structure can also be used as an infrared detector at a low temperature (77° K.). Since PtSi has a resistivity of 30μΩ-cm, it has been suggested for use as a gate and interconnection metal over polysilicon. However, the high temperature stability of PtSi has always been questionable. 
     The often quoted high temperature instability of PtSi on silicon apparently arises from the fact that the lowest eutectic point in the Pt-Si system occurs at 830° C. Even after the complete consumption of metal to form silicide at a low temperature, an agglomeration and resistance increase at a temperature of 700° C. or higher has been reported. The high temperature instability of the PtSi has also been observed in the degradation of the PtSi/n-Si Schottky diode. Annealing such diodes to a temperature of 700° C. or higher will cause large reverse leakage current and non-ideal forward current-voltage characteristics with lowered Schottky barrier height and an ideality factor much larger than unity. This is due to the reason that the Pt from the PtSi has diffused into the silicon substrate. 
     Up to the present, the only method that can slightly improve the high temperature instability associated with the PtSi/Si structure is to produce a film with a Si/Pt atomic ratio higher than that in the eutectic to bypass the eutectic composition and hence to improve the stability. This method is achieved by co-sputtering silicon rich alloy. The co-sputtered silicon rich alloy prevents the agglomeration of the platinum silicide, but increases the resistivity and decreases the Schottky barrier height of the film. Electrical properties, such as forward and reverse current-voltage characteristics, of the PtSi/Si structure still degrade after high temperature annealing. Furthermore, this method is not compatible with the modern VLSI process. 
     SUMMARY OF THE INVENTION 
     It is therefore one object of the present invention to provide a method of improving the high temperature stability of PtSi/Si structure. 
     It is another object of the present invention to provide a method of adjusting the Schottky barrier height of the PtSi/Si Schottky contact. 
     It is the final object of the present invention to incorporate PtSi into an integrated circuit during an early or middle stage of the fabrication process of VLSI so as to improve the devices and/or circuit performance. 
     The above objects can be achieved by implanting sufficient amount of fluorine-contained ions into the PtSi/Si structure. Generally, as long as the dose of the fluorine ion penetrated into the Si substrate exceeds 1×10 14  cm -2 , the PtSi/Si structure obtained can be subjected to an annealing temperature higher than 800° C., and the Schottky barrier can be adjusted to as high as 0.984 eV. 
     Further objects, features and advantages of this invention will appear more fully in the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be explained in more detail on the basis of an exemplary embodiment with reference to the drawings, in which: 
     FIG. 1 is a diagram showing the sheet resistance of the F +  implanted and unimplanted PtSi/Si samples versus annealing temperature; 
     FIG. 2A is a SEM micrograph of the silicide-stripped silicon surface of a F +  implanted sample; 
     FIG. 2B is a SEM micrograph of the silicide-stripped silicon surface of an un-implanted sample; 
     FIG. 3 is a diagram showing the RBS profiles of the PtSi/Si structure with F +  implantation followed by 400° and 800° C. annealing, respectively, and the profile of the PtSi/Si structure annealed at 800° C. without F +  implantation; 
     FIG. 4 is a diagram showing the ideality factor and barrier height of the F +  implanted and unimplanted PtSi/n-Si samples versus annealing temperature; 
     FIG. 5 is a diagram showing the F +  ion distribution in the PtSi(60 nm)/p-Si sample with 30 keV F +  implantation (the implantation dose is 2×10 15  cm -2 ) obtained by using TRIM-86 Monte Carlo simulation; 
     FIG. 6 is a diagram showing the distributions of Pt and F of samples having PtSi(60 nm)/n-Si structure, obtained by using SIMS. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     According to the method of the present invention, the fluorine-contained ions can be directly implanted into a PtSi/Si structure prepared by co-deposition of Pt and Si substances on a Si substrate or by direct reaction of Pt/Si at a low temperature or can be implanted into a Pt/Si structure followed by high temperature annealing or can be implanted into a Si substrate followed by Pt or PtSi film deposition and high temperature annealing. In the following paragraphs, the method of the present invention will be described with respect to the implantation of fluorine-contained ions into a Pt/Si structure followed by high temperature annealing. 
     The starting material was a crystalline n-type (100) oriented 4-10 Ω-cm phosphorus doped silicon wafer. Following the standard RCA cleaning process, a 470 nm oxide was thermally grown and a junction area of 1000×1000 μm 2  was defined by a photolithographic technique and BOE etching. A Pt film of 30 nm thickness was then deposited at a rate of 0.1 nm/sec in an e-beam evaporation system with a base pressure below 4×10 -6  torr. Fluorine ions (F + ) were implanted to a dose of 2×10 15  cm -2  at 30 KeV (some wafers were implanted by BF 2   +  ions to a dose of 5×10 15  cm -2  at 80 KeV) followed by a 400° C. furnace annealing in N 2  atmosphere for 60 min. Unreacted Pt was selectively etched by dilute hot aqua regia. Wafers were further annealed at higher temperatures ranging from 700° to 850° C. for 90 min in a N 2  atmosphere. 
     FIG. 1 shows the sheet resistance (R S ) of the F +  implanted and unimplanted PtSi/n-Si samples after annealing at various temperatures. It is apparent that the high temperature stability is preserved up to 800° C. for the F +  implanted samples. SEM micrograph of the PtSi-stripped silicon surface after 800° C. annealing with and without fluorine implantation are shown in FIG. 2A and FIG. 2B, respectively. It is obvious that the F +  implantation can prevent the PtSi film from agglomeration and prevent Pt from diffusing into the silicon substrate efficiently. Rutherford backscattering spectroscopy (RBS) also shows that for the F +  implanted sample the Pt atom does not penetrate into the silicon substrate after an 800° C. annealing, while it has penetrated deeply for the unimplanted sample at 800° C., as illustrated in FIG. 3. 
     Schottky diode characteristics of the F +  implanted and unimplanted PtSi/n-Si sample were also examined. FIG. 4 shows the ideality factor (n) and the Schottky barrier height (φ bn ) of the F +  implanted and unimplanted PtSi/n-Si diodes annealed at various temperatures. For the unimplanted diodes, the barrier height decrease and the ideality factor increases at an annealing temperature higher than 700° C. However, for the F +  implanted diodes, the barrier height increase to 0.984 eV at 800° C. annealing temperature and may be varied by adjusting the annealing temperature around 800° C., while the ideality factor remains at an acceptable value (lower than 1.05). 
     The mechanism by which the fluorine ion improves the high temperature stability and varies the Schottky barrier can be explained by the &#34;Fluorine Barrier Model.&#34; During the high temperature annealing process, the implanted fluorine ion out-diffuses and piles up at the PtSi/Si interface. This phenomenon is correlated with the strong binding energy of the Si-F bond (5.73 eV) such that the fluorine atom can be easily bonded to the dangling bond of Si at the PtSi/Si interface. If the concentration of the accumulated fluorine layer is high enough, it acts as a barrier to prevent transport of Pt into the silicon substrate. Thus, even after annealing at a temperature as high as 800° C., the PtSi/Si-Si structure remains stable. Since the fluorine atoms are bonded to the dangling bond of Si at the PtSi/Si-Si interface, the surface states are substantially diminished. Thus, the Schottky barrier height is dominated by the difference of the work function of PtSi and the electron affinity of Si. This explains the barrier variation phenomenon. 
     The minimum F +  ion dose of 1×10 14  cm -2 , which must be incorporated into the Si substrate for improving the high temperature stability of PtSi/Si structure, applies to a Si substrate of both impurity types and various doping concentration. BF 2   +  was implanted into Pt/n-Si or PtSi/n-Si substrate to form a PtSi/p +  n shallow junction, and F +  and As +  were also respectively implanted into a Pt/p-Si or PtSi/p-Si substrate to form a PtSi/n +  p shallow junction; in both cases, the same result of high temperature stability was obtained. It is also to be noted that the minimum F +  ion dose, i.e. 1×10 14  cm -2 , is refers to the amount of F +  ion that is penetrated into the silicon substrate. This amount is calculated by a TRIM-86 Monte Carlo simulation based on the implantation energy and the thickness of Pt or PtSi of the Pt/Si or PtSi/Si structure. FIG. 5 is a diagram showing the F +  ion distribution of the PtSi (60 nm)/p-Si sample which is implanted with F +  ions at 30 KeV to a dose of 2×10 15  cm -2 . From this, the concentration of F +  ions implanted into the Si substrate was calculated to be 1.2×10 14  cm -2 . Furthermore, as shown in FIG. 5, the concentration of F +  ions implanted into the Si substrate is the highest at the surface of the silicon substrate, i.e. at the PtSi/Si interface, and decreases gradually with increasing depth. This is a suitable and effective ion implantation. If the energy for the ion implantation is too small such that most of the F +  ions can not enter the Si substrate, no effect can be obtained. However, if the energy for the ion implantation is too large such that most of the F +  ions penetrate deeply into the Si substrate away from the PtSi/Si interface, also no effect can be obtained. This is because the promotion of high temperature stability of the PtSi/Si structure by the F +  ion implantation is due to the pileup of F +   ion at the PtSi/Si interface and the F +  ion&#39;s bonding to the dangling bond of Si atom at the Si surface to form Si-F bonds so as to prevent the Pt atoms in the PtSi film from diffusing into the Si substrate. FIG. 6 is a diagram showing the distributions of F and Pt of the samples prepared by implanting BF 2   +  ion into a PtSi (60 nm)/n-Si structure followed by high temperature annealing. This diagram is obtained by using Secondary Ion Mass Spectroscopy (SIMS). From this diagram, it is clear that during annealing, the F +  ion has a tendency to pile up at the PtSi/Si interface. However, if the F +  ion is implanted too deep such that the F +  ion cannot be piled up at the PtSi/Si Pt interface in time, no good result can be expected. It is also known from FIG. 6 that 80 keV is an appropriate implantation energy, while 40 keV is too small to obtain the desired effect and therefore Pt diffuses into Si substrate in a large amount after annealing at 800° C. The energy for implantation depends on the species of the fluorine-containing ion used (such as F +  or BF 2   + ) and the thickness of Pt in the Pt/Si structure or the thickness of PtSi in the PtSi/Si structure. 
     The apparatus employed for carrying out the method of the present invention, such as the ion implanter, and the fluorine-contained source, such as F +  or BF 2   + , are very commonly used in semiconductor industry and therefore the method of the present invention can be easily performed in a general semiconductor fabrication factory. 
     While this invention has been described with respect to a preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.